This is not meant to be a complete survey of CO2 concentrations occurring in Yellowstone's thermal areas. My goal was to locate potential field sites for plant physiology studies.

For this reason, the following priorities were considered when taking measurements:

--Unvegetated areas were not measured.

--Areas containing less than 400ppm CO2 were not recorded unless it seemed that they could serve as control sites for other areas containing the same plant and soil types.

--Areas containing diffuse CO2 emissions that covered a large surface area were preferred over areas that contained smaller vents with localized emissions or emissions that were strongly affected by prevailing winds. Relatively stable CO2 concentrations were preferred over episodic peaks.

--Areas containing previously un-surveyed vegetation or soil types were targeted over areas similar to those I had already visited.

Survey Procedure:

I walked slowly across an area, dragging the CO2 meter's intake hose behind me or dropping it in attractive-looking areas until the meter indicated above-ambient CO2 levels. Then I would spiral outward, using the yellow flags to indicate areas of highest concentration. Unless otherwise noted in the site descriptions, CO2 levels are above ambient only in the areas surrounding the yellow flags. In general, CO2 emmissions were more diffuse (covered a larger surface area) in the following locations: Mammoth Upper Terraces, Mud Volcano and surroundings, Sulphur Mountain, Forest Springs and Sylvan Springs. The flags in these locations are only representative of quite large areas that contain above-normal CO2. In general, the thermal areas along the Firehole River (e.g. Lone Star group, Old Faithful, Pocket Basin, Twin Buttes) contained more localized CO2 emissions.

The following areas had no suitable study sites, even though they did have CO2 vents: Culex Basin, Porcupine Hills, The Quagmire Group, The Ferris Fork of the Bechler River, Vermillion Springs, Ebro Springs, and West Thumb.